Storage circuit for ferroelectric display screen



Nov. 22, 1966 H. cox, JR., ETAL 3,287,600

STORAGE CIRCUIT FOR FERROELECTRIC DISPLAY SCREEN 2 Sheets-Sheet 1 FiledNov. 19, 1962 Fig, 3

INVENTO S- HENRY L. COX, JR. BY JURIS A. ASARS +Eg I |+E 4 5 4. ATTORNEYFig/ 22, 1956 H. L. cox, JR.. ETAL 3,287,600

STORAGE cmcun' FOR FERROELECTRIC DISPLAY SCREEN Filed Nov. 19. 1962 Fl 4R g. 7 input ZTutput 2 Sheets-Sheet 2 Fig. 4.4 W U IT rmrmrm 119 I L LUIJL L R iR i T RI T.- HEW.

. i Q Z 7 Y A A T T Y3 T T T INVENTORS.

1/ ATTORNEY United States Patent M 3,287,600 STORAGE CIRCUIT FORFERROELECTRIC DISPLAY SCREEN Henry L. Cox, Jr., Baltimore, Md., andJuris A. Asars,

Monroeville, Pa., assignors, by mesne assignments, to

the United States of America as represented by the Secretary of the NavyFiled Nov. 19, 1962, Ser. No. 238,812 1 Claim. (Cl. 315-169) Thisinvention relates to display systems, and more particularly to displaysystems having a panel-like screen structure which is formed offerroelectric ceramic cells or elements.

The most common display systems utilize a conventional cathode ray tubeby which an electron beam is used for supplying energy for producing thelight output from a phosphor display screen and also for distributingthe video information over the screen area. In a scanned display, theelectron beam of the cathode ray tube excites a particular point on thephosphor screen once every scan period and the decay time of thephosphor and the persistence of the eye combine to produce theimpression of a continuous light output from the screen. Cathode raytubes are deficient at times particularly in that their use does notprovide adequate screen brightness and additionally produces a degree offlicker and obiectional line structure in the image provided for theviewer.

One object of this invention resides in improved circuitry for supplyinginformation to a display screen which is formed of ferroelectric cellsor elements in order to provide for continuity of light output over theentire display screen for a desired time interval.

Another object resides in improved capacitatiye storage circuits fortransferring information to a display screen formed of ferroelectriccells or elements.

A more specific object resides in an arrangement of an air gap or ionicswitch for the charge capacitative storage circuits which is effectiveto establish a potential on the ferroelectric elements corresponding to.the information signal.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings, wherein:

FIG. 1 shows the outline of a display screen with. a representativegroup of ferroelectric cells incorporated therein and shown greatlyenlarged;

FIG. 2 is a greatly enlarged detail of structure which forms the air gapor switch for the charge capacitative storage circuits oftheferroelectric cells or elements;

FIG. 3 is a diagram showing the potential current characteristics of theair gap or switch of FIG. 2;

FIG. 4 is a schematic showing of the capacitative storage circuit;

FIGS. 4a and 4b are potential diagrams for the input and outputrespectively of the capacitative storage circuit of FIG. 4;

FIG. 5 is a schematic showing of a group of capacitative storagecircuits of FIG. 4 arranged for supplying information to and storinginformation in the representative group of cells of FIG. 1.

Referring to FIG. 1, the display screen will be understood to be formedof a large number of small ferroelectric elements or cells 11, -a smallgroup of which are shown greatly enlarged in the figure. In an actualmodel, a screen of approximately 4" x 8" contained 8192 cells or 256cells to the square inch and provided an effective operating displaystructure. Regardless of the actual number of elements used to form ascreen, the functions of the elements or cells 11 are those of emittinglight in accordance with the AC. potential appear- 3,287,600 PatentedNov. 22, 1966 ing across a cell and of causing this potential to bealtered in accordance with the applied video control potential.

An article by E. A. Sack published in the Proceedings of IRE forOctober, 1958, at page 1695, describes one type of screen in which theferroelectric cells were developed from the ceramic form of bariumstrontium titanat'e and the electroluminescence utilized in the screenwas of the intrinsic type consisting of a phosphor powder for emittinglight under the action of a changing electric field. The cells orelements were mounted on a laminate of transparent material and althoughthey were barely recognizable due to their miniature size, the screenprovided in effect a configuration of electroluminescent andferroelectric capacitors.

In another article by E. A. Sack and others published in the April, 1962issue of Proceedings of the IRE, the operation of an electroluminescentferroelectric display screen is explained in some detail. By way ofgeneral explanation of the operation of such a display screen, it may bestated that information is supplied to one row of cells at a time,starting at the top and stepping consecutively toward the bottom of thescreen. The 4"x8 screen of FIG. 1 would thus have 64 horizontal rows ofcells formed in 128 vertical columns. Target information in the form ofacomputer word is fed from an information source to a translator whichgenerates the characters for the display screen. The translator selectsfrom its memory all of the targets which touch the top row of the screen10 and generates a spatial geometry of signals which represent thetarget character forms. Information for the first row of the screen isthen fed to apparatus known as display column drivers and immediatelythereafter the translator supplies a row select signal to apparatusknown as the row drivers for triggering the signals into the top row ofthe screen. The cycle is then repeated for each successive row until theentire panel has been scanned and all targets which fall into the panelarea are displayed. Due to the excellent storage characteristics of thescreen, the target images may be retained until further updateinformation becomes available.

Pulse actuated switches play an important role in signal distributionsystems for electroluminescent ferroelectric display screens. In thesesystems the information signal in the form of DC. potentials istransferred to the cells or elements 11 of the display screen by pulseactuated switches and the DC. potential retained on the cells orelements 11 until, new information for those elements is received. Sincethe cells or elements 11 function as capacitors for storing information,the pulse actuated switch must be capable of changing the charge on acapacitor during a short time interval and retain it for a much longerperiod.

In FIG. 2 is shown structure which forms one type of switch in the formof an air gap termed an ionic switch and which has been operatedsuccessfully in signal distribution. The disclosure of FIG. 2 shows theswitch structure enlarged substantially and in unassembled condition asa convenient way of explaining the switch construction. Each ionicswitch assembly is fabricated from two mirror halves. As shown in FIG.2, beveled steatite carriers 15 are employed as supports for electrodes16-16 which have a gap G therebetween. The electrodes are formed, forexample, of platinumiridium metal ribbon and during fabrication a groupof the ribbons 16 are attached by a layer 17 of epoxy resin to a patternof the steatite carriers 15. The resulting composite structure is thenseparated by cutting the ribhens and subassemblies, as shown in FIG. 2,are provided from which air gaps or ionic switches can be produced. Thesurfaces 18 on each of the carriers 15-15 provide a reference planewhich permits the two subassemblies to be secured together with theirsurfaces 18-18 in contact to provide air gaps G having the desiredspacings between the electrodes 16-16 and a switch structure will beprovided for each of the cells 11 of the screen.

Known capacitative storage circuits utilizing ionic switches or sparkgaps have required high switching potentials and large amounts of energyto actuate the switch and transfer information to the display screen.The diagram of FIG. 3 is for the purpose of illustrating the potentialcurrent characteristics of an air gap or ionic switch of the type shownin FIG. 2. Consider 'this air gap or switch as being formed between thetwo platinum-iridium electrodes 16 of 0.05 mm. cross section and the gapformed therebetween to be 5 microns. The switching burst or initialbreakdown potential for the gap G is indicated at E, and within therange of approximately 350 to 400 volts. The constant sustainingpotential indicated at E, aind which is maintained across the gap afterthe initial breakdown is approximately 330 volts for currents up tomilliamperes. With such structure, variations in gap spacing of fromabout 2 to 8 microns will change the sustaining potential by only a fewvolts.

FIG. 4 shows a capacitative storage circuit utilizing an air gap orionic switch, of the type shown in FIG. 2, for operating the cells orelements 11 of the display screen. The circuit consists of a resistor R,which represents source resistance as well as any other requiredresistances of the input, the gap G formed between the platinium-iridiumelectrodes 16 and a storage capacitor C. In FIG. 4a, the input to thiscircuit is supplied with the information potentials which vary between Eand E, which is less than the 330 volts sustaining voltage on theelectrodes 1616. At the desired time the switching burst of between 350and 400 volts is superimposed on the information potential to be storedon capacitor C and after the termination of the burst this potentialremains on the storage capacitor C until a new burst is applied.

As shown in FIG. 4a, the switching burst consists of a number of squarewave cycles with short overshoots or spikes superimposed at all leadingedges. To obtain bidirectional clam-ping action, the peak-to-peakamplitude of the square wave component is equal to 2 E which causes thegap to conduct current and equalize the average potential across it. Ifthe average potentials of both sides of the gap are the same, thepositive circuit resistance, R, limits the amount of current to a verysmall value. Amplitude of the spikes, B is somewhat larger than thebreakdown potential, E of the gap to insure reliable switching. Thesespikes are used only to initiate breakdown of the gap during each halfcycle and transfer only a very small amount of charge because of theirshort duration. The difference in amplitude between E',, and E, isrequired to minimize the spread of stored potentials caused by thestatistical behavior of gaps.

FIG. 5 shows a group of air gap or ionic switch storage circuits of FIG.2 which correspond in a number to the representative group of cells 11of FIG. 1. The information potentials are supplied by bus lines X X andX to the air gaps G of the associated storage circuits on a time sharebasis while the swtiching bursts are supplied simultaneously to thecapacitors C of the storage circuits by the bus lines Y Y and Y Anadditional resistor R is also provided in each information circuit inorder to eliminate interaction between the information and burstpotentials, while an isolation resis tor R is provided in the outputline 0 of each storage circuit. It will be understood that the outputlines 0 of each storage circuit is connected to its corresponding cell11 of the display screen 10.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claim the invention maybe practiced otherwise than as specifically described.

We claim:

In a display system utilizing a display screen formed ofelectroluminescent ferroelectric signal storage elements which aredisposed in rows and columns to be scanned by information signals, theimprovement comprising,

(a) first bus lines for supplying information potential to said signalstorage elements,

(b) second bus lines for supplying switching potential to said signalstorage elements,

(c) a storage circuit for each storage element comprising in series, aresistance, an air gap and a capacitor interconnecting said first andsecond bus lines and (d) said air gap being formed between twoelectrodes References Cited by the Examiner UNITED STATES PATENTS 9/1955Anderson 340173.2 9/1955 Anderson 340173.2 11/1958 Young 340173.211/1958 Stadler 340-173.2 12/1959 Sack 313l08 6/1961 Drougard et al.340173.2

JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner.

